A variety of services are available today for inhouse cleaning of carpets and upholstery. These services utilize equipment for heating cleaning liquid which is conveyed under pressure to and sprayed onto the surface to be cleaned and then vacuum removed from the surface with the soil. This equipment, which often includes an internal combustion engine for driving the cleaning liquid and vacuum pumps, is usually mounted in a panel truck, or van, for ease of transport.
It has been suggested that instead of using a separate heater for heating the cleaning liquid that waste heat from the internal combustion engine be used for that purpose. U.S. Pat. No. 4,593,753, granted June 10, 1986 to P. J. McConnell for "EXHAUST GAS LIQUID HEATING SYSTEM FOR INTERNAL COMBUSTION ENGINES" discloses a system for heating water with exhaust gas heat. U.S. Pat. No. 4,109,340 granted Aug. 29, 1978 to L. E. Bates for "TRUCK MOUNTED CARPET CLEANING MACHINE" discloses a system in which the cleaning liquid is passed first through the cylinder block of a liquid cooled, internal combustion engine and then through a heat exchanger which also has engine exhaust gases passing therethrough. U.S. Pat. No. 4,284,127 granted Aug. 18, 1981 to D. S. Collier et al for "CARPET CLEANING SYSTEMS" discloses a similar system which directs the cleaning liquid through a first heat exchanger into which the liquid engine coolant also is directed. The preheated cleaning liquid then passes through a second heat exchanger where it extracts heat from the engine exhaust gases.
In all of the aforementioned systems in which the cleaning liquid is directed in heat exchange relationship with the exhaust gases of the internal combustion engine there is a danger that the cleaning liquid could become overheated. To avoid damage to surfaces to be cleaned the temperature of the cleaning liquid, as a general rule, should not exceed 220.degree. F. Internal combustion engine exhaust gases can reach temperatures as high as 1200.degree. F. With the engine running and a low flow rate for the cleaning liquid the latter can rapidly be heated to an undesirably high temperature in the exhaust gas heat exchange. It has been customary, therefore, to incorporate into such systems a thermostatically controlled dump valve for dumping the overheated cleaning liquid before it can reach the surface to be cleaned. One such dumping arrangement is described hereinafter and in the aforementioned co-pending application Ser. No. 07/286,616.
U.S. Pat. No. 3,594,849 granted July 27, 1971 to C. L. Coshow for "CLEANING APPARATUS" discloses a cleaning system in which air and heated cleaning fluid being withdrawn from a carpet is conveyed in heat exchange relationship with cleaning liquid being conveyed to the carpet. Little benefit, if any, is derived from this relationship a it would seem to decrease, rather than increase the temperature of the cleaning liquid being conveyed to the carpet.
The aforementioned co-pending application Ser. No. 07/286,616 propose utilization of the heat contained in the return air stream after it has passed through the vacuum pump. Because the vacuum pump adds a significant quantity of heat to this air stream useful heat can be obtained from its exhaust and imparted to the cleaning liquid being heated. This feature of that application is carried over and constitutes a part of the invention covered by the present application. It should be noted, however, that co-pending application Ser. No. 07/286,616 offered no suggestions for preventing overheating of the cleaning liquid in heat exchange relationship with the engine exhaust gases.